The present invention relates generally to imaging apparatus and methods, and more particularly, to apparatus and a method for maintaining a substantially constant closely spaced working distance between a nozzle plate of a ink jet print head and a surface of a medium or element for receiving liquid or ink droplets as the droplets are being deposited on the receiving medium or element during relative movement of the print head and the medium or element.
Ink jet imaging devices use the controlled ejection of small droplets of liquid, to produce an image. Typically, the liquid is ejected through one or more nozzle orifices located in a nozzle plate of a print head. The ejection of the liquid, for instance, ink, through the respective nozzles is effected by a pressure pulse, which in the instance of a piezoelectric print head, is generated by application of an electrical drive waveform to an electromechanical transducer, and in a thermal print head, by application of a waveform to an electrothermal transducer or a resistor.
A problem with known ink jet printing apparatus is that ink droplets ejected from a nozzle orifice may emerge or travel in a direction that varies from the intended direction which is usually perpendicular to the surface of the nozzle plate. Such misdirection can arise from physical causes including a nozzle imperfection or a deposit or deposits on a surface of the nozzle, and can result in an error in the final location of the dot produced by the ink droplet on the receiving medium or element with respect to a desired or intended location of the dot. Such locational errors can result in artifacts in the printed image, such as visible bands and the like.
One contemplated solution for decreasing the severity of such locational errors in dot placement is to reduce the distance between the nozzle plate and the ink receiving medium or element, which is referred to as the working distance. This solution may be beneficial for applications requiring very high image quality and dot placement accuracy, as for example graphic arts printing, in which the spatial frequency of the micro-dots forming the image may be very high: for instance, 1200-2400 dots/inch or higher. In several prior art printing applications, requiring lesser dot placement accuracies, the working distance may be set relatively large, for instance, ink jet printers typically used for home and business applications wherein the working distance is generally 1 to 1 xc2xd millimeters to accommodate varying paper thicknesses. However, for other applications in which it is desirable to set the working distance to a much smaller value, for example, on the order of 100 to 1000 microns, decreasing the working distance poses the danger of collision or contact between the print head and the receiver, which can result in damage to or destruction of the print head and/or the receiver. Also, the liquid, when ejected from a nozzle typically consists of a liquid droplet having a connected ligament or trailing tail. Some distance of travel from the ejecting nozzle is required for the liquid structure to coalesce into a single or unitary droplet desirable for producing a satisfactory dot. Therefore, the working distance is desired to be small, but not so small as to provide insufficient distance above the receiver surface for coalescence. If the working distance is not sufficiently large to provide the required travel distance, the liquid objects will impinge the surface of the ink receiving medium or element before being completely formed which may result in comet shaped or other undesirable marks on the surface. And, if the working distance is not maintained substantially constant, variations in the printed dots can be present.
To compound the problems in maintaining a substantially constant working distance, typically during ink jet printing, the print head and ink receiving medium or element are moving transversely one relative to the other in at least one direction or along at least one axis. For instance, the ink receiving medium or element will be moved or translated in a direction indicated as y, while the print head maybe moved or scanned across the receiving medium or element in a direction indicated as x. Velocities of movement, for example in the instance of large format printing apparatus, can range up to about 1 meter per second. Such movement can result in variations in the working distance between the print head and the surface of the receiving medium or element due to any of several factors, including, for instance, variances in thickness of the ink receiving medium or element and/or non-flatness of the ink receiving surface thereof, such as a that due to a bow in a platen used for vacuum hold-down of a receiver for printing, or imperfections in the transport or support apparatus for moving the print head and/or the receiving medium or element one relative to the other, for instance, in the case of a cylindrical or roll shaped receiving medium, an eccentricity which results in variations of the working distance when the medium is rotated.
Various methods and apparatus are known for moving and positioning print heads or nozzles for depositing ink and other materials onto surfaces of substrates and other receivers. Reference in this regard, Hirano et al. U.S. Pat. No. 5,468,076 issued Nov. 21, 1995 to Kabushiki Kaisha Tec of Japan which discloses several embodiments of a print gap adjusting device; Petermann U.S. Pat. No. 5,360,276 issued Nov. 1, 1994 to Siemens Nixdorf Informationssysteme Aktiengesellschaft of Germany which discloses a printing device with adjustable printing head gap; and Kotsuzumi et al. U.S. Pat. No. 4,652,153 issued Mar. 24, 1987 which discloses a wire dot-matrix printer. However, all of these devices are directed to the problem of statically sensing the thickness of a receiver or paper placed on a platen, and then adjusting a print head-to-platen gap accordingly, prior to printing. This is in contrast to the present invention, which dynamically senses a print head-to-receiver gap and uses the sensed signal to keep this gap constant, during printing. In addition, all the above cited devices rely either on contact with the paper or other printing receiver or stored data to determine the print gap value to be used. Reliance on contact with the receiver can be a disadvantage in that wear, dirt build-up, and/or marring or other marking of the receiver surface or ink or other material previously deposited on the surface can occur due to the contact. And, reliance on stored gap values may be disadvantageous if the values do not correspond to the actual value required for a particular instance.
Reference also, Tylko U.S. Pat. No. 5,894,036 issued Apr. 13, 1999, which discloses a three dimensional plotter which maintains an ink droplet, or bead, by coordinating the delivery rate of the ink through the dispensing nozzle with the dispensing nozzle height and velocity; and Batcheleder U.S. Pat. No. 5, 303,141, issued Apr. 12, 1994 to International Business Machines Corporation which discloses a model generation system having closedxe2x80x94loop extruding nozzle positioning which includes apparatus for generating a feedback signal that is indicative of at least one characteristic of a most recently extruded portion of material extruded through the nozzle, which apparatus can include, for instance, a visual or infrared emission imaging system, a proximity detecting apparatus such as a capacitive sensor, a tactile sensor, or a pneumatic sensor. However, these disclosures refer to methods for coating a material like a slurry, or adhesive, onto a surface, in which a continuous-pressure pump is used to maintain a contiguous bead of coating material, between the nozzle and the receiver. This is in contrast to the case of the present invention, in which a drop of liquid is broken off from the nozzle by a discontinuous pressure pulse, and travels to the receiver. In addition, these referenced apparatus and methods do not disclose a means for maintaining a substantially constant closely spaced working distance between an ink jet print head and a surface of a receiver for receiving ink droplets therefrom during a printing operation wherein a print head and the receiver are moved one relative to the other.
Therefore, there is a need to provide apparatus and a method for maintaining a substantially constant closely spaced working distance between an ink jet print head and an ink receiving surface during a printing operation wherein the ink receiving medium or element may vary in thickness and/or flatness, and/or the transport system for moving the print head and/or the receiver may be imperfect or imperfectly aligned, and which apparatus and method does not necessitate reliance on contact with the receiver or predetermined gap values.
An object of the present invention is to provide apparatus and a method for maintaining a substantially constant closely spaced working distance between an ink jet print head and an ink receiving printing medium or element during printing and which does not require contact with the receiver or reliance on stored distance values.
With this object in view, the present invention resides in an apparatus and system for maintaining a print head at a substantially constant, closely spaced working distance from a surface of a printing receiver during relative translational movement between the print head and the receiver along a predetermined path of movement while liquid or ink droplets are being deposited onto the surface by the print head, the apparatus including an actuator connected to the print head and controllably actuable for moving the print head toward and away from the receiver, an element for detecting a representative distance between the print head and locations on the surface of the receiver along the path of movement and generating a signal having a value representative of the representative distance, an element for comparing the signal value to a reference value representative of the working distance for determining a difference between the signal value and the reference value and generating a control signal representative of the difference, and a control element for receiving the control signal and responsively controllably actuating the actuator to move the print head toward or away from the receiver to locate the print head at the working distance from the respective locations on the surface of the receiver when the print head is positioned for depositing the liquid or ink droplets thereon.
The present invention also resides in a method for maintaining the substantially constant closely spaced working distance between the ink jet print head and the surface of the printing receiver as the print head and the receiver are moved transversely one relative to the other along the path of movement, and the liquid or ink droplets are being deposited onto the surface including the steps of
(a) generating signals representative of distances between the print head and locations on the surface of the receiver along the path of movement prior to the print head being positioned for depositing liquid or ink droplets onto the locations on the surface, respectively; and
(b) responsively to the signals, as required, moving the print head toward or away from the receiver to locate the print head at substantially the closely spaced working distance from the locations on the surface of the receiver when positioned for depositing the liquid or ink droplets thereon, respectively.
According to an exemplary embodiment of the present invention, the receiver is translated in one direction, while the print head may be scanned across the receiver in another direction. The element for detecting the representative distance between the print head and the locations on the surface of the receiver and generating the signal representative of the representative distance comprises a capacitive circuit wherein one plate of the capacitor is formed by a conductive portion or layer of the printing receiver and the other plate is a component of the print head, for example, an electrode attached to the print head, or a nozzle plate of the print head having conductive properties. In this way, as the print head and the printing receiver are relatively moved, the capacitance will vary according to the distance and circuitry is provided to generate a corresponding signal which will be representative of the changing distance. Circuitry including a comparator is provided for receiving the signal representative of the representative distance and comparing it to a known or predetermined value for the desired working distance for generating the control signal, the control signal being provided to the control element for controllably actuating the actuator. The actuator, for example, can be a stepper motor, which turns a threaded shaft connected to the print head, the control element comprising a stepper motor drive. A biasing element, for example, a spring, can be used for biasing the movement of the print head for reducing hysteresis and backlash in the threaded movement of the print head. As an alternative, for instance, wherein the printing receiver does not possess conductive properties, the element for detecting the distance between the print head and the locations on the surface of the receiver along the path of movement can include another sensor construction such as an optical distance sensor.
A feature of the present invention is the provision of an ink jet printer including apparatus for maintaining a substantially constant closely spaced working distance between the print head thereof and a printing receiver.
Another feature of the invention is the provision of a feedback system adapted for determining representative distances between the print head and locations on a surface of the receiver onto which liquid or ink droplets will be deposited during the printing operation and moving the print head toward or away from the receiver to locate the print head at the desired working distance from the receiver when positioned for depositing the droplets on the respective surface locations.
As an advantage of the present invention a substantially constant closely spaced working distance is maintained between the ink jet print head and the printing receiver for more accurately produced dots on the receiver.
As another advantage, variances in printed dots and artifacts due to an inconsistent working distance can be reduced.
According to the invention, a substantially constant closely spaced working distance is maintained between an ink jet print head and an ink receiving printing medium or element resulting in improved print quality with less artifacts.
These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there are shown and described illustrative embodiments of the invention.